We had everything de-normalized, as you pointed out.
However, every now and then we had some one-off questions that required using Mongo for something it's not designed for. However, migrating to postgres+JSONB made it easy to do both.
We've been on a big "Use postgres for everything" path. Mostly migrating dynamodb, elasticsearch, and redis to postgres.
The number of times we've had to do some one-off query that was orthogonal to production usage is higher than I predicted. There were so many times in the past where we just didn't fix something because it was too difficult to backfill a document store. We just hacked around it.
Now with postgres we fix the root of the problem instead of hacking around it. And we do so with a level of confidence we've never had with document stores.
However, every now and then we had some one-off questions that required using Mongo for something it's not designed for. However, migrating to postgres+JSONB made it easy to do both.